1. Field of the Invention
The present invention relates to prosthetic devices used to replace diseased or damaged spinal discs.
2. General Background and State of the Art
The adult spine has 26 vertebrae (depending how one counts) with fibrocartilage, intervertebral discs between adjacent vertebrae. The vertebrae include seven cervical vertebrae in the neck, 12 thoracic vertebrae below the neck, five lumbar vertebrae of the lower back, one sacrum below the lumbar region and one coccyx, or tailbone. The discs form strong joints, separate, cushion and allow flexure and torsion between the vertebrae.
When functioning properly, the vertebrae and discs allow a person to bend forward, backward and to the sides and to twist. To accomplish this, the discs permit adjacent vertebrae six degrees of motion: vertical (compressing to absorb shock and tension), bending forward and backward, bending to the sides and twisting. The cervical and lumbar discs also can be thicker anteriorly to contribute to lordosis. Thoracic discs are more uniform. Unfortunately, disc disease limits spinal motion or cushioning or only permits the motion with pain.
Each intervetebral disc has a central area composed of a colloidal gel, called the nucleus pulposus, on a surrounding collagen-fiber composite structure, the annulus fibrosus. The nucleus pulposus occupies 25-40% of the disc's total cross-sectional area. The nucleus pulposus usually contains 70-90% water by weight and mechanically functions like an incompressible hydrostatic material. The annulus fibrosis surrounds the nucleus pulposus and resists torsional and bending forces applied to the disc. The annulus fibrosis thus serves as the disc's main stabilizing structure. The annulus fibrosus resists hoop stresses due to compressive loads and the bending and torsional stresses produced by a person bending and twisting. The fibers of the annulus form lamellae, individual layers of parallel collagen fibers, that attach to the superior and inferior end plates of adjacent vertebrae. Vertebral end-plates separate the disc from the vertebral bodies on either side of the disc.
The anterior longitudinal ligament, which is anterior to the vertebral bodies, and the posterior longitudinal ligament, which is posterior to the vertebral bodies and anterior to the spinal cord hold the spinal structure together. The muscles of the trunk provide additional support.
Trauma or disease may displace or damage spinal discs. A disc herniation occurs when annulus fibers weaken, and the inner tissue of the nucleus bulges out of the annulus. The herniated nucleus can compress a spinal nerve, which results in pain, loss of muscle control or even paralysis. Alternatively, disc degeneration results when the nucleus deflates. Subsequently, the height of the nucleus decreases causing the annulus to buckle in areas where the laminated plies are loosely bonded. This also causes chronic and severe back pain.
Whenever the nuclear tissue is herniated or the disc degenerates, the disc space narrows and the adjacent vertebra may lose much of their normal stability. In many cases, to alleviate pain from degenerated or herniated discs, a surgeon removes the nucleus or the disc as a whole and fuses the two adjacent vertebrae together. While this treatment usually alleviates the pain, the patient loses all disc motion in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent to the fused segment as the adjacent discs compensate for lack of motion. The added stress may lead to premature degeneration of those adjacent discs. See, Lehman, T. R., Spratt, K. F., Tozzi J. E., et al., “Long-Term Follow-Up of Lower Lumbar Fusion Patients.” Spine (1987) 12:97-104.
Surgeons have replaced damaged discs with prosthetic devices. The devices generally take four approaches, hydraulic, elastic, mechanical, and composite. Traynelis V. and Haid R. “Spinal Disc Replacement: The Development of Artificial Discs.” Spine Universe (Oct. 10, 2001), gives an overview of the state of the art as of 2001.
A useful disc prosthesis must satisfy several criteria. It must maintain proper spacing between adjacent vertebrae, permit desired motion, provide stability and absorb shock. The disc prosthesis should not shift axial load significantly from where a natural disc would apply those loads. Any disc prosthesis should replicate normal ranges of motion from front to back, side to side, vertically and in torsion. The prosthesis also must constrain motion.
Any disc prosthesis must retain its initial functional characteristics for many years and over many cycles. Studies estimate that a typical patient with a prosthesis will need it for 50 years. Replacing a worn-out prosthesis should be avoided. The average persons takes 2 million strides annually and bends over 125,000 times per year. Thus, any prosthesis cycles more than 100 million times in 50 years.
The prosthesis materials must be biocompatible and not corrode. It also should not inflame surrounding tissue. Because of the millions of cycles to which the prosthesis material will be subjected, the material must have a high fatigue strength. It also should give off minimal debris. Some believe that the prosthesis also should show up well on X-ray images. Likewise, the prosthesis must be capable of fixation to bone. In addition, the design should protect against a catastrophic failure from the failure of any individual component. In addition, the prosthesis should guard against damage to surrounding tissues, particularly the spinal cord.
The Link SB Charité disc from Waldemar Link GmbH & Co of Germany has been widely implanted. It consists of an ultra high molecular weight polyethylene spacer between two endplates. The Charité disc provides only three degrees of freedom and no load support. It allows for flexion and extension, lateral bending and rotation but not compressive movement or lateral or sagittal shear.
The Bryan cervical disc from Spinal Dynamics Corp. of Seattle, Wash. uses an elastic nucleus between two metal plates. A flexible membrane between the plates surrounds the nucleus. The Bryan disc also provides five degrees of freedom and only partial load support. It allows for flexion and extension, rotation, lateral bending and lateral and sagittal shear. This disc is recommended only as a replacement for cervical discs.
Ray, U.S. Pat. No. 5,824,093 (1998), also is an example of the elastic type of disc prosthesis. The patent discloses a disc prosthesis having upper and lower end plates with a constraining jacket and a deformable gel core. End plates, which attach to adjacent vertebrae, apply load to the gel causing the gel to deform. Friction causes one problem in this device. The gel is encapsulated, and as it deforms, the expansion takes place laterally along the insides of the top and bottom surfaces. Over time, as the surfaces of the gel capsule and top and bottom plates change, the friction changes. This causes the prosthetic disc to function improperly.
The hydrogel disc replacement is an example of the hydraulic approach. A hydrogel implant replace only the nucleus; the annulus fibrosis is not replaced. Raymedica, Inc. of Bloomington, Minn., manufactures a PDN prosthetic disc nucleus implant that consists of a woven polyethylene jacket constraining a hydrogel core. The jacket is flexible but inelastic. Therefore, the jacket allows the hydrogel core to deform and reform as it is loaded and unloaded, but the jacket limits the core's horizontal and vertical expansion
Some are concerned that prostheses that use gel or polymer will lose the resiliency or the resiliency will change over the many cycles of loading and unloading. Others are concerned that as the molecules of the resilient material move relative to each other and move with respect to the material encapsulating the gel, some molecules or groups of molecules will break off and enter surrounding tissue. The body recognizes these molecules as foreign bodies and attacks them. This biological activity can cause the prosthesis to lose its grip with the surrounding bone tissue.
Instead of a gel for resiliency, several patents propose using springs. For example, Patil, U.S. Pat. No. 4,309,777 (1982), has springs spaced around the periphery of opposing cups. However, Patil also fails to provide natural motion for adjacent vertebrae. Beer, U.S. Pat. No. 5,458,642 (1995), spaces the springs laterally. Ratron, U.S. Pat. No. 5,676,702 (1997), relies on a specially shaped resilient member. Larsen, U.S. Pat. No. 5,782,832 (1998), has a linkage between the top and bottom plates with resilient springs between those plates.
Butterman, U.S. Pat. No. 5,827,328 (1998), suggests having different springs for different embodiments of its invention. Mehdizadeh, U.S. Pat. No. 5,928,284 (1999), shows a disc prosthesis that threads between vertebrae. Springs push apart parts of the threaded member. Ralph, U.S. Pat. No. 5,989,291 (1999), uses Belleville washers for its resiliency. Finally, Pisharodi, U.S. Pat. No. 5,123,926 (1992), spring-biases the spikes used to hold the prosthesis in place. The springs also expand the prosthesis.
Each of these devices is problematical. When prior art prostheses use coil springs, adjacent coil windings can touch or rub against each other. If the coils touch and depending on how they touch, the touching can create a sound, which is unnatural and can be unpleasant. Further, when adjacent coils rub against each other, microscopic pieces of metal can rub off the spring. The body can attack these pieces and create an immune reaction, which can loosen the spring. Using a plastic spring does not solve the problem because plastic molecules also can rub off. Further, plastic may not be strong enough for the small springs necessary. Fortunately, one can manufacture small springs with the necessary spring constants with this application. Further, one can design metal springs that maintain the same spring constant over the anticipated useful life of the prosthesis.
Using springs can be advantageous. They are reliable. Springs are intrinsically stable and designed for cyclic loading. However, prior art spring prostheses fail to consider proper spring design. In particular, the diameter of the spring wire and the way in which the wire is wound affect the spring's mechanical properties. Moreover, the spring must be limited in size. The spring must have a diameter no greater than the outside dimensions of adjacent vertebrae, and the spring must cause the prosthesis to be no taller than the disc being replaced.
Prior art metal spring prosthesis usually have alignment problems. If the applied force is not aligned with the axis of the spring, the spring may cant. These problems can cause the springs to work unevenly.
The patient's height and weight and the particular disc to be replaced affect the size and properties of the spring. Thus, for example, the spring constant and disc configuration are very different for a prosthesis to replace the disc between the forth and fifth lumbar vertebra in a 6′2″, 200 lb. (188 cm, 91 kg) male then for the disc between the third and fourth thoracic vertebra in a 5′2″, 115 lb. (157 cm, 52 kg) female., (Metric/English conversions are approximate.) Using the correct spring or group of springs replicates the functions of a healthy disc. Having many different versions of specially configured springs available for a surgery can be costly, however. In addition, fabrication costs for specialized springs are greater.
Surgical procedures for disc replacement are very complex and subject to many complications. State of the art prostheses contribute to the complexity of disc replacement surgery. Aligning and securing present prostheses can be very difficult and time consuming.